The present invention relates generally to the field of MODFETs (modulation doped heterojunction field effect transistors) on n-AlGaAs/GaAs structures for analog operation, particularly at millimeter wavelengths.
Since the discovery by Dingle et al ("Electronic Mobilities in Modulation-doped Semiconductor Heterojunction Superlattices", Appl. Phys. Lett. Vol. 33, p.665, (1978)) of high mobility two-dimensional electron gas in N-AlGaAs/GaAs heterojunction structures, considerable progress has been made in utilizing these structures for practical realization of FETs based on Schottky-barrier type gates. See for example S. Hiyamizu et al. "MBE-Grown GaAs/n-GaAs Heterostructures and Their Application to High Electron Mobility Transistors", Japan J.Appl. Phys. Vol 21, p.161, (1981); D. Delagebeaudeuf et al, Metal-(n)AlGaAs-GaAs Two Dimensional Electron Gas FET", IEEE Trans Electron Dev. ED-29, p.955, (1982); T. J. Drumond et al, "Influence of Al.sub.x Ga.sub.1-x As Buffer Layers on the Performance of Modulation Doped Field Effect Transistors", Appl. Phys. Lett. Vol. 40, p.879, (1982); and P. C. Chao et al "Quarter-micron Gate Length Microwave High Electron Mobility Transistors", Presented at the 1983 IEEE/Cornell Conference on High Speed Semiconductor Devices and Circuits", Ithaca NY, Aug. 15-17, 1983. For a review paper on MODFETs, see P. M. Solomon and H. Morcoc, IEEE Trans. on Electron Devices, ED-31, p.1015 (1984). Also of interest is a paper by M. Ogura et al. "Self-aligned Enhancement Mode FET with AlGaAs as Gate Insulator", J. Vac. Sci. Technol. B 3 (2), Mar/Apr 1985, page 581.
U.S. patents of interest relating to field effect transistors include U.S. Pat. No. 4,494,016 to Ransom et al disclosing a GaAs depletion mode MESFET useful as a linear power amplifier. It includes a semi-insulating host GaAs substrate which has a uniform depth N+ source, gate and drain implant. The N+ implant region has a very high dopant concentration. Delagebeudeuf et al in U.S. Pat. No. 4,455,564 are concerned with a high frequency field effect transistor which uses a heterojunction between the GaAs and AlGaAs layers to create a high mobility electron accumulation layer and a Schottky grid to control the load. Decker et al in U.S. Pat. No. 4,157,556 disclose a heterojunction in a confinement field effect transistor. In that patent a GaAs conducting channel is bounded by an AlGaAs layer. Cho et al in U.S. Pat. No. 4,186,410 discuss the formation of ohmic contacts in GaAs FETs. Ohmic contacts using silicon doped aluminum are taught in Morcon et al U.S. Pat. No. 4,333,100. Yoshida et al in U.S. Pat. No. 4,492,974 disclose a DMOS which comprises a MOSFET with a gate protection diode.